This invention relates to automatic protection switching systems and, more particularly, to a protection switching system in which service lines and/or offices may be easily and economically added.
In communications systems, networks including service lines are employed to transmit signals between locations, for example, telephone offices. Some of these offices may be located intermediate to others of the offices. In modern communications systems, it is desirable to employ service lines which link the intermediate offices with each other and also service lines which by-pass one or more of the intermediate offices in linking others of the offices in the system together. In such systems, transmission reliability must be maintained at a "high" level. A desired reliability is realized by employing some form of protection against failure of service lines linking the intermediate offices directly as well as those service lines which by-pass one or more of the intermediate offices in linking others of the offices together. Heretofore, protection was generally provided on a so-called point-to-point basis. Consequently, separate protection lines or channels were provided for service lines directly linking intermediate offices and for service lines which by-pass one or more intermediate offices. Such protection arrangements are undesirable from both an economic as well as a system growth basis. Indeed, such protection systems require duplication of equipment and do not allow for easy and economical addition of new lines and/or offices to the overall communications system.
In one known system, the problems, of the point-to-point protection switching systems are overcome by employing a first protection line between a first terminal and an intermediate office and a second protection line between the intermediate office and a second terminal. The first and second protection lines are arranged to be cascaded controllably to provide protection for service lines directly linking the first and second terminals in addition to providing protection for service lines linking the intermediate office with each of the terminals. The protection lines employed in this prior arrangement are of a type described in an article authored by H. D. Griffiths and J. Nedelka entitled "100A Protection Switching System" Bell System Technical Journal, December, 1965, page 2295. Cascading of the protection lines is achieved by employing a switch arrangement at the intermediate office. Voice frequency signals transmitted over a separate transmission facility are employed in the 100A system to control the intermediate switching arrangement for cascading the protection lines. When the protection lines are cascaded they provide protection for the lines directly linking the first and second terminals. When the protection lines are not cascaded, they are available to protect the associated service lines between the intermediate office and the first and second terminals. This prior known system is capable only of providing protection between the two terminals and one intermediate office. It is not readily expandable to include other intermediate offices and/or terminals without substantial re-engineering and substantial system modifications to effect desired cascading of multiple protection lines. The need for additional engineering and modification of equipment is undesirable. Moreover, use of a separate transmission facility to transmit signals to and from each terminal and the intermediate office in order to effect the desired switching is also undesirable.
More recently, a system has been proposed for providing alternate transmission channels when a channel fails. The alternate channels are provided by switching transmission from the failed channel to an alternate channel which may include one or more transmission links. The switching is controlled, however, from a central control center. Specifically, switch requests are transmitted to the control center where the availability of an alternate channel is checked and appropriate commands issued. Consequently, each switch location must be connected via a communications link to the control center. Such central control arrangements are hierarchical and usually dictate that the entire system topology and the status of each switch in the system be known at the control center. Furthermore, the control center must be appropriately programmed to effect the desired switching to alternate channels. Consequently, changes in the system topology may require alteration of the communications system between the control center and switch locations as well as reprogramming of the control center itself.
Thus, although prior known protection switching arrangements may function satisfactorily in some applications they are undesirable for others because of their limitations. Moreover, because of the rapid growth experienced in communications systems, it is highly desirable to deploy a system including protection switching in which both service line additions between existing offices and new office additions are easily and economically realized.